Unit 2 / Matter

Chemical Groups/Families

Introduction:

If we look at the total number of elements which exist, we can say that relatively few basic kinds of matter are found (about 100). However, these elements are chemically combined into thousands and thousands of different compounds. How does a chemist study such an overwhelming number of separate facts? Do they memorize all the possibilities? NO WAY! Scientists have discovered a number of patterns of behavior among the different chemical elements and have developed theories to account for these patterns. These patterns and their explanations help us to understand not only the elements themselves, but also the countless compounds that are formed from them.

Patterns of Behavior:

Let us first consider the most basic chemical property of the elements: chemical activity/reactivity. There are 6 elements which either do not show chemical activity or show it very weakly - the noble gases. They are rare except for Ar which is third in abundance in the atmosphere. The chemical inactivity of the noble gases indicates a basic similarity among the 6 elements, therefore it is not surprising that they share a number of common properties.

The most distinctive property of the group of elements is that at room temperature they are all gases. Furthermore, they all exist as monoatomic gases. In 1894, John Rayleigh and William Ramsay isolated a small bubble of air and subjected it to every known chemical test. When no reactions were observed, they concluded that they had discovered a new element which was chemically inert (inactive). They named the element argon from the Greek word meaning "lazy". Ramsay and co-workers discovered three more rare gases during 1894-98:

  • neon from the Greek word meaning “new”
  • krypton from the Greek word meaning “hidden”
  • xenon from the Greek word meaning “strange”

He Ne Ar Kr Xe Rn

Atomic Mass (amu) 4.0 20.2 39.9 83.8 131.3 222

Melting Point (*C) --249-189-157-112-110

Boiling Point (*C) -269-246-186-153-107 -62

Hfusion (Kcal/mol) - 0.08 0.27 0.36 0.49 0.80

Hvaporization (Kcal/mol) 0.02 0.44 1.50 2.31 3.27 3.92

1.Can you detect some patterns in the properties of the 6 elements with increasing atomic mass?

  1. Suppose scientists were to discover another inactive element having an atomic mass greater than that of radon. What conclusions could you draw about the element? Do you think it would be a solid, liquid, or gas at room temperature? How would its boiling point compare to radon’s?
  • By knowing a certain pattern exists within a group of elements, it is possible to predict the properties of any member of that group from the properties of any other member. These patterns developed very slowly over the history of chemistry, but today the knowledge that elements fall into related groups is one of the most valuable tools of the chemist.
  • We will study other significant groups/families that exist in order to demonstrate further the occurrence of patterns/similarities among the chemical elements.

The Halogens (Group VIIA):

  • 4 elements which are strikingly different from the noble gases in their physical and chemical properties are the following diatomic elements:

1.Fluorine (F2) - a violently active, pale-yellow gas that can only be prepared and handled safely under strictly controlled conditions. It is the most reactive element.

2.Chlorine (Cl2) - a dense, choking, poisonous, greenish-yellow gas of high chemical activity.

3.Bromine (Br2) - a red-brown, highly corrosive, volatile liquid at room temp.

4. Iodine (I2) - beautiful purple-black crystals that sublime when heated to yield a violet vapor. Moderately reactive.

  • these 4 elements have properties that are closely related. A variety of experiments done to study these properties indicate that they can be grouped as a chemical family. One of these properties is that each of the elements reacts with metals to form salts. The family name halogens comes

from the Greek word for “salt-formers.”

metal + halogen ---> salt (halide)

examples: 2Na(s) + Cl2(g) ---> 2NaCl(s)

2K(s) + I2(s) ---> 2KI(s)

  • all the “free” halogens are active non-metals which are toxic and corrosive to the skin. They are much too reactive to remain in nature in free form. They will quickly combine with other elements to form compounds.
  • the fifth halogen is called astatine from the Greek word meaning “unstable.” Astatine is a man-made/synthetic element which was first made in 1940 at U. Cal.-Berkeley. Astatine has no stable non-radioactive form so little is known of its physical and chemical properties by direct experimentation. Could you predict some of those properties?

F2 Cl2 Br2 I2 At

Atomic Mass (amu) 38.0 71.0 159.8 253.8 ?

Boiling Point (*C) -188 -34 58 184

Melting Point (*C) -220-102 -7 114

Hfusion (Kcal/mol) 0.12 1.53 2.52 3.74

Hvaporization (Kcal/mol) 0.56 4.88 7.18 9.98

Formula of sodium halide NaF NaCl NaBr NaI

Uses:

1. Fluorine

a.Freons (chlorofluorocarbons) are used as refrigerants and foaming agents.

b.Teflon - a plastic (polymer) which is composed of large molecules containing fluorine and carbon.

c.NaF, in very small quantities, is used to “fluoridate” public water supplies.

At a concentration of 1 part per million, F- ions are effective in preventing tooth decay. Many toothpastes contain NaF or SnF2 for the same reason.

2. Chlorine

a.As a bleaching agent of textiles and wood pulp. laundry bleach: 2NaOH(aq) + Cl2(g) ---> NaOCl(aq) + NaCl(aq) + H2O(l)

b.Purification of water - added at low concentrations to drinking water and swimming pools to kill algae and bacteria.

c.Useful in the industrial preparation of a large number of important chemical compounds (e.g. HCl and plastics such as polyvinylchloride).

3. Bromine

a.AgBr - a light-sensitive compound which allows the development of photographic film.

4.Iodine

a.AgI - also a light-sensitive compound used in photographic film.

b.When dissolved in alcohol solution (tincture of iodine), used as an antiseptic.

c.Iodide ions, in small amounts, are essential to our diet. They are required to form the growth-regulating hormone, thyroxine, C15H11O4NI4. This compound is produced by the thyroid gland. If the gland receives too little iodine, the condition of goiter will result. To prevent this, about 0.01% of sodium iodide, NaI, is added to ordinary table salt to give “iodized salt.”

Alkali Metals (Group IA):

  • a family of soft, silvery-white metals that are very active chemically. They must be stored either in a dry, inert atmosphere or under oil, since they all react quickly with oxygen and violently with water. In their pure form, they must be handled with great care.
  • are much too reactive to occur as free metals. They are found as +1 ions in ionic compounds.
  • francium is a man-made radioactive element and does not occur in nature so little is known about its chemistry.
  • called the alkali metals because many of the compounds they form are strong alkalies (bases).

2K(s) + 2H2O(l) ---> 2KOH(aq) + H2(g) strong base

4Na(s) + O2(g) ---> 2Na2O(s) metal oxide

Na2O(s) + H2O(l) ---> 2NaOH(aq) strong base

  • react with halogens to form halide salts

example: 2K(s) + Cl2(g) ---> 2KCl(s)

Li Na K Rb Cs

Atomic Mass (amu) 6.9 23.0 39.1 85.5 132.9

Boiling Point (*C) 1331 890 766 701 685

Melting Point (*C) 181 98 63 39 29

Hfusion (Kcal/mol) 0.72 0.62 0.55 0.52 0.50

Hvaporization (Kcal/mol) 32.2 21.3 18.5 16.5 15.8

Density (g/mL) 0.53 0.97 0.86 1.52 1.87

Alkaline Earth Metals (Group IIA):

  • also are too reactive to occur free in nature. They are found as +2 ions in ionic compounds such as carbonates (M2+, CO32-) or sulfates (M2+, SO42-).
  • the most abundant alkaline earth compound occurs in the earth’s crust. CaCO3 makes up limestone, marble, and seashells among others.
  • calcium is required in our diets because it is important in the growth and maintenance of healthy bones. A long term deficiency in calcium can lead to a severe bone condition called osteoporosis.
  • magnesium ions have a fairly high concentration in seawater.
  • the lightest member and heavier members of Group 2 are much less abundant than magnesium and calcium.
  • beryllium is rare, but can be found in an ore called beryl (Be3Al2Si6O18) which makes up the precious stones emerald and aquamarine. Traces of impurities give these gems a green or light blue color.
  • strontium and barium occur as both the carbonate (SrCO3, BaCO3) and sulfate (SrSO4, BaSO4).
  • radium compounds are extremely rare. The element was first isolated by Marie Curie in 1898. She obtained a fraction of a gram of radium from more than a ton of pitchblende ore. Radium is radioactive and compounds of the element have been used for many years in the treatment of cancer.

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Chemistry I Cary Academy W.G. Rushin